Radio power amplifiers are used in telecommunications, for example, both for transmission and for reception. To operate a power amplifier efficiently, with maximum power and gain, it is desirable to run it with bias voltage and input signal amplitude conditions such that it functions close to saturation, with the consequence that the linearity of its transfer function is degraded or distorted relative to an otherwise similar amplifier running further from saturation.
In order to compensate for the distortion, the amplifier is conveniently linearised by a pre-distorter, which should be simple to use and easy to integrate on the same semiconductor chip as at least initial stages of the main amplifier module, the so-called monolithic integrated circuit (‘MIC’) technology. Such a pre-distorter presents to the input signal of the amplifier a non-linear transfer function that tends to compensate the non-linearities in the operation of the power amplifier module. The pre-distorter comprises a scaled amplifier of the same physical characteristics, in particular the same technology and process of manufacture as the amplifier to linearise, whose component elements are easy to integrate and which will require a low number of tuning operations.
Many ways of building pre-distorters have already been described and many of them use a “linear/non-linear path” topology. One linear/non-linear path topology is described in patent specification U.S. Pat. No. 4,992,754 and comprises a first path providing a copy of the input signal (linear path) and a second path in which some distortion is introduced through a non-linear device (non-linear path). The difference between the signals from the two paths contains a distortion that is arranged to be in opposition with the distortion of the power amplifier to linearise, in order to cancel out the distortion introduced by the power amplifier.
One limitation of that system is that some delay must be introduced in the linear path to compensate for the delay in the non-linear path, and the relationship between the two paths in phase and amplitude is not easily maintained with frequency and from part to part. Another limitation is in the couplers, which may be well suited for hybrid technology but which are not easy to integrate, especially on semiconductor material such as Silicon where transmission lines are lossy.
U.S. Pat. No. 5,576,660 describes a pre-distorter that comprises a 90° input divider and a 90° output coupler in hybrid form, the circuits of the two channels (or paths) being interconnected by micro strip circuits or by hybrid techniques, and it cannot readily be implemented in integrated circuits. The two paths of the pre-distorter include microwave amplifiers, attenuators and variable phase shifters that are adjusted to adapt the transfer characteristics to a power amplifier of different technology. From the user point of view, numerous tuning operations are necessary in order to optimise the linearity of this system. Phase and gain need to be adjusted in each path to adjust the shape of the gain and phase pre-distortion curves. The bias voltages of all the active devices used in both paths need to be adjusted, and also the total gain of the pre-distorter to adjust its level characteristic to that of the power amplifier (the level at which the amplifier will start to introduce distortion).
The present inventor's previous patent specification EP 022 913 305 describes a pre-distorter that is integrated in MIC technology and is more readily manufactured and tuned. However, it is still desirable to improve the characteristics relating to variability of resistors and their non-linearity when used at high RF level, to simplify further the tuning operations and to reduce further overall losses of the complete pre-distorter.